Optimization in KDE Base

The problem was that we drawed the overlays using KIconLoader, which can
be very slow, every time an item appeared on the screen. This commit
makes sure that not only the icon, but the icon including overlays is
cached in QPixmapCache. Therefore, the overlay drawing is done just once
for each icon+overlays combination.

For previews, the overlay drawing is done in KFileItemModelRolesUpdater
just after the preview is received.